In such a shelving system it is generally necessary to load or unload a storage space of a stack through an aisle separating it from an adjoining stack, e.g. in order to transfer a loaded pallet from one stack to the other. This is particularly desirable where shipments are to be assembled from the contents of several storage spaces on the two stacks, as is frequently the case in large-scale industrial warehousing.
It is known, e.g. from U.S. Pat. No. 3,608,749, to operate a stacker crane in an aisle between high-rising storage racks, such a crane having a platform with a bidirectionally extendable arm which can reach into the storage spaces on either side of the aisle to pick up a pallet for removal to an unloading station or to deposit a pallet picked up at a loading station. If the pallets are stacked two deep on either side of the aisle, the extendability of the arm in either direction must be slightly greater than two pallet lengths. If the depth of the aisle also slightly exceeds two pallet lenghts, the stacker crane can carry a pair of pallets simultaneously extracted from or to be deposited in a rack.
If, in such a system, the arm is to load or unload only a single storage space proximal to the aisle, the corresponding pallet comes to rest on one of the extremities of that arm so as to be deliverable to another storage space on the same side of the aisle but not on the opposite aisle. This limited maneuverability of the stacker crane complicates the filling of orders which must be assembled from goods stored on both sides of the aisle.